Freshly planted trees and shrubs will require additional attention to watering as they become established. As the roots grow deeper into the soil, they become less dependent on water available in the topsoil and irrigation becomes less critical. Heavy mulching of garden beds around trees and shrubs will minimise irrigation. The watering requirements of shrubs and trees varies enormously from species to species. For example, many Australian or South African plants may require no additional watering once established, while many exotics demand more water than is naturally available in order to survive.
The water and feeding needs of fruit trees are normally greater than ornamental plants of similar size. Mixing plenty of organic matter into the soil prior to planting will not only help the plants access nutrients, but will also help the soil retain moisture, reducing the need for watering. Keep grass and competing weeds clear of root zones and again mulch heavily to conserve water.
Growing vegetables need between 25 and 50 mm of water per week for good growth. If watering by hand or with a sprinkler, a watering about once a week so that the water soaks to a depth of about 150 mm will often be adequate, repeated again when the top few mm of soil have started to dry out. A vegetable garden can be effectively watered by sprinklers, microsprays or drippers, but if using sprinklers, remember the need to avoid too much water standing on foliage, particularly overnight.
Groups of plants in containers may be watered as a group by sprinklers or micro-sprays, but the most efficient solution is to use drip irrigation, with individual running to each pot. Each dripper is fitted to the end of a piece of flexible feeder tube connected to a main polytube feeder. The flexible feeder tubes allow the pots to be moved about or changed at will. The flow rate of the drippers should be adjusted to suit the size and water needs of each plant.
Capillary pot watering enables a large number of pots to be watered simultaneously. Pots sit on a porous mat through which water is circulated, to be drawn into the pots by capillary action. This labour-saving technique is best suited to nurseries and large greenhouses, with no wastage of water through run-off.
Avoid standing pots in saucers or on a surface which may block the drainage holes or on which rainwater will collect and stand. Container plants require moisture to drain freely through the potting mix, otherwise their roots will rot. The solution can be found in small ‘feet’ which raise the pot from the surface, permitting free drainage. Water-absorbing granules mixed into the potting mix reduces the risk of plants drying out and allows longer spells between watering.
Plants grown in hanging containers, particular those made of porous organic materials, are particularly vulnerable to moisture loss as well as being easily forgotten. Like other container plants, they are best watered by a system of feeder lines fixed to the overhead structure with drippers on flexible tubes running into each basket.